Add open-source maintenance tools

Add contribution guidelines for open-source project

Explain Claude Code and provide links in READMEs

Add installation instructions for recommended document skills

Recommend Anthropic core document skills in READMEs

Enforce no em-dashes across all marketing skills

Enhance marketing skills with human-first writing and viral LinkedIn frameworks

Add Salesably Marketplace logo to README

Add sales-skills plugin with 9 specialized sales skills

Update marketplace.json to match official schema

Add skills path to plugin.json

Fix marketplace name to match repo

Add marketplace README

Initial commit: marketing-skills plugin